1. Field of the Invention
The present invention relates to an idle control device for an engine, and in particular to an idle control device using torque-based control.
2. Description of the Related Art
In engine control devices of the recent years, a method of controlling an engine called “torque-based control” has been getting popular, in which torque produced by the engine is controlled based on a torque index, as described in Patent Document 1, for example.
In such torque-based control as described above, target torque of the engine is determined based on the quantity of stepping on an accelerator pedal by a driver, the opening degree of a throttle valve is controlled so that the engine draws in an air volume capable of producing the target torque, and an ignition coil is controlled at an ignition timing capable of realizing the target torque with an actual intake air volume detected by an air flow sensor, whereby output power from the engine is controlled to produce the target torque, so as to realize running performance requested by the driver.
Moreover, when the accelerator pedal is not stepped on, that is, when the engine is in idling, the total of loss torque of the engine (mechanical loss and pump loss) and that of accessory loads (such as an alternator and an air conditioner) are determined to be the target torque, the opening degree of the throttle valve is controlled so that the engine draws in the air volume capable of producing the target torque, and the ignition coil is controlled at an ignition timing capable of realizing the target torque with the actual intake air volume detected by the air flow sensor, whereby the output power from the engine is controlled to produce the target torque, so as to realize a target revolution speed.
In the torque-based control as described above, in the computing process, the target torque needs to be converted into the intake air volume and the ignition timing; the conversion methods therefor are such that “ignition-timing-engine-torque characteristics” that have been measured in advance on the basis of an internal combustion engine operation state (a revolution speed and an intake air volume into the cylinder) are stored as map data in the memory of the control device, and then the intake air volume is obtained from the torque, the revolution speed and the ignition timing, or the ignition timing is obtained from the torque, the revolution speed and the intake air volume.
However, the relationship between the ignition timing and the torque is characterized as approximating a quadratic function; therefore, the farther an actual ignition timing is set from the ignition timing that gives the maximum engine torque (the vertex of the quadratic function; referred to as the MBT ignition timing), that is, the farther it is set from the vertex of the quadratic function, the wider a torque variation range tends to become relative to an ignition timing variation range.
An example of “ignition-timing-torque characteristics” is shown in FIG. 9 for reference.
In FIG. 9, when a change or shift of an ignition timing ΔIG occurs at an ignition timing relatively close to the MBT ignition timing, a torque change or torque shift of ΔT2 occurs. Meanwhile, when the change or shift of the ignition timing ΔIG occurs at an ignition timing relatively far from the MBT ignition timing, a torque change or torque shift of ΔT1 (>ΔT2) occurs.
As described above, it is understood that the farther the ignition timing shifts from the MBT ignition timing, that is, the more the ignition timing is set toward the retard side, the more the torque shift expands attributed to the “ignition-timing-torque characteristics.”
Moreover, as changes in the characteristics of various parts progress over time, a discrepancy between the “ignition-timing-torque characteristics” stored in advance in the memory of the control device and “ignition-timing-produced-torque characteristics” as actual engine characteristics becomes greater; therefore, a discrepancy between torque actually produced by the engine and the target torque increases even at the same ignition timing.
When an air volume is calculated under such conditions, from torque, a revolution speed and an ignition timing in idling, if the air volume calculated from the map data of the “ignition-timing-torque characteristics” is larger than a necessary volume, actual torque becomes excessive, so that the revolution speed increases exceeding a target idle revolution speed, whereas, if the air volume calculated from the map data of the “ignition-timing-torque characteristics” is smaller than the necessary volume, the actual torque becomes short, so that the revolution speed becomes lower than the target idle revolution speed.
Moreover, when an ignition timing is calculated from the torque, the revolution speed and the air volume, if the ignition timing calculated from the map data of the “ignition-timing-torque characteristics” is advanced too much, the actual torque becomes excessive, so that the revolution speed increases exceeding the target idle revolution speed, whereas, if the ignition timing calculated from the map data of the “ignition-timing-torque characteristics” is retarded too much, the actual torque becomes short, so that the revolution speed becomes lower than the target idle revolution speed.
However, in either of the cases described above, by providing additional control such as PID control based on a revolution deviation of the actual revolution speed from the target revolution speed and by adding to the target torque a correction value based on the revolution deviation of the actual revolution speed from the target revolution speed, excess or shortage of actually produced torque is fed back and corrected so that the intake air volume is increased or decreased; however in particular, when the ignition timing is greatly shifted toward the retard side, the revolution speed decreases so much that return to the target revolution speed will be delayed, which raises concern in that unpleasant engine noise arises, or in the worst case, the engine might be led to a stall.
That is to say, when a discrepancy arises in the “ignition-timing-torque characteristics,” if the ignition timing is normally advanced in response to a reduction in the revolution speed so as to increase torque, the return to the target revolution speed can be brought forward; however, in the torque-based control described above, an ignition timing to achieve the target torque is set based on the actual intake air volume, and an increase in engine torque to compensate the reduction in revolution speed is not directly reflected on the ignition timing, so that the increase in the engine torque is delayed, thereby delaying the return to the target revolution speed.
As countermeasures against the above-described problems, there are known a method of setting a predetermined restriction position in advance and making the ignition timing not be set in the retard side beyond the predetermined restriction position, as described in Patent Document 2, and another method of making the ignition timing not be shifted toward the retard side when the revolution speed is lowered beyond the target idle revolution speed, as described in Patent Document 3. However, even if those methods described in Patent Document 2 and 3 are unchangingly implemented in the above-described torque-based control, delay in the return to the target revolution speed is not resolved for reasons as below.
Patent Document 1: Japanese Patent No. 3627464
Patent Document 2: Japanese Patent No. 3556682
Patent Document 3: Japanese Patent No. 3435760
The above-described torque-based control is configured in such a way that when the target torque becomes unable to be realized at an ignition timing computed based on the intake air volume, excess or shortage of the target torque that becomes unable to be realized at the ignition timing is controlled to be fed back to the target air volume. Therefore, if the ignition timing is restricted before retarded to a target value, the target torque is corrected so that the target intake air volume is corrected toward a decreasing side, and resultantly, regardless of the ignition timing being restricted from shifting toward the retard side, the intake air volume is controlled to change toward the decreasing side, so that torque that cannot be realized at the ignition timing (difference between torque able to be produced at the target ignition timing and that able to be produced at the restricted ignition timing) is increased late, which has caused a problem in that delay in the return to the target revolution speed cannot be resolved.